Mutations in the gene encoding the ATM serine-threonine kinase cause Ataxia Telangiectasia (A-T), a disease marked by lymphopenia and an increased incidence of lymphoid tumors with translocations involving antigen receptor loci, suggesting that ATM functions during V(D)J recombination. ATM activates cell cycle checkpoints in response to DNA double strand breaks (DSBs). However, the lymphoid phenotypes of A-T are not recapitulated in mice with isolated deficiencies in checkpoint pathways. We have demonstrated that ATM functions to repair DSBs generated during V(D)J recombination, and to suppress the aberrant resolution of these DSBs as chromosomal translocations. The combined defect in checkpoint pathways and DSB repair explains some of the lymphoid phenotypes of A-T. Given all of the defects in V(D)J recombination observed in ATM-deficient lymphocytes, we have proposed that ATM functions, in part, to maintain the stability of DSB complexes after RAG-mediated DNA cleavage, which is a hypothesis that will be directly tested in Specific Aim One. Although ATM could function directly in the repair of RAG-mediated DSBs, we expect that ATM will likely phosphorylate proteins that perform this function. In this regard we show that the MRN complex (Mre11, Rad50 and Nbs1), 53BP1 and H2AX, which are all targets of ATM, function in the response to RAG-mediated DSBs. How these proteins function in the ATM-dependent pathway of RAG-DSB repair will be elucidated in Specific Aim Two. In addition, we will consider the possibility that the RAG proteins may have ATM-dependent functions in the joining step of the V(D)J recombination reaction. Importantly, we believe that these different proteins will function in an integrated manner in the ATM-dependent RAG-DSB repair pathway. Finally, we will investigate the mechanisms by which DNA DSBs generated during V(D)J recombination in ATM-deficient cells become aberrantly resolved as chromosomal translocations (Specific Aim 3). The completion of these aims will provide important new information about: 1) how RAG-mediated DSBs are repaired; 2) how ATM functions in DSB repair and in maintaining genomic stability; and 3) the mechanisms that promote the formation of chromosomal translocations.